The role of FSH in male reproduction has not been fully clarified. An important objective of our research is to examine the biochemical and morphological responses of the testis and epididymis to withdrawal of FSH without resorting to hypophysectomy. This is achieved through the use of a relatively new, simple and elegant tool, namely antigonadotropic sera. The most important initial aspect of this work is to make absolutely certain that our anti-FSH serum binds I125 FSH and not I125 LH. The rationale behind our approach is to selectively reduce endogenous FSH without altering circulating testosterone values. Our preliminary experiments in immature rats reveal that selective withdrawal of FSH leads to a serious reduction in spermatogenesis. We are now pursuing similar experiments in the adult rat. Since FSH acts on Sertoli cells an important aspect of our work will be to examine the morphology and secretory activity of Sertoli cells following FSH depletion. We are also utilizing anti-LH serum to reduce testosterone levels. The aim of this portion of the work is to dissect the relative roles of FSH and testosterone, respectively, on spermatogenic function. BIBLIOGRAPHIC REFERENCES: Dym, M., Madhwa Raj, H.G. and H.E. Chemes (1977) Response of the testis to selective withdrawal of LH or FSH using antigonadotropic sera. In: New Concepts of the Testis in Normal and Infertile Men: Morphology, Physiology and Pathology, Raven Press, New York, In press. Martin Dym and H.G. Madhwa Raj, Response of the adult rat testis to selective depletion of luteinizing hormone (LH). Proceedings of the annual meeting of the Endocrine Society, 1977. In press.